Feature: Saved developer searches with new-match alerts

[sajeetharan]

Problem

Developers repeat the same searches by language, location, and contribution profile, but cannot save criteria or learn when new matches appear.

Proposal

Allow signed-in users to save a search and optionally receive an alert when newly indexed public developers match it.

Acceptance criteria

• Save, rename, run, and delete search criteria.
• Support current text/vector mode and structured filters.
• New-match detection is incremental and deduplicated.
• Alerts are opt-in with per-search frequency controls.
• Private or pending profiles never appear.

Related

• Feature: Restore recent discovery session and saved context #120